The invention relates generally to machining of an advanced material, and in particular to hybrid machining a titanium article, such as an airfoil of a gas turbine engine.
Today's manufacturing industry is facing challenges from advanced difficult-to-machine materials (tough super alloys, ceramics, and composites), stringent design requirements (high precision, complex shapes, and high surface quality), machine, and machining costs.
Advanced materials play an increasingly important role in modern manufacturing industries, especially, in aircraft, automobile, tool, die and mold making industries. The greatly-improved thermal, chemical, and mechanical properties of the material (such as improved strength, heat resistance, wear resistance, and corrosion resistance), while having yielded enormous economic benefits to manufacturing industries through improved product performance and product design, are making traditional machining processes unable to machine them or unable to machine them economically. This is because traditional machining is most often based on removing material using tools harder than the workpieces.
In addition to the advanced materials, stringent design requirements also pose major problems in manufacturing industry. More and more complex shapes (such as an aerofoil section of a turbine blade, complex cavities in dies and molds, non-circular, small, and curved holes), low rigidity structure, and micromechanical components with tight tolerances and fine surface quality are often needed. Traditional machining is often ineffective in machining these parts. To meet these challenges, new processes have been developed.
Electrical discharge machining (or EDM) is a machining method primarily used for hard metals or those that would be impossible to machine with traditional techniques. One critical limitation, however, is that EDM only works with materials that are electrically conductive. EDM can cut small or odd-shaped angles, intricate contours or cavities in extremely hard steel and exotic metals such as titanium, hastelloy, kovar, inconel and carbide.
Sometimes referred to as spark machining or spark eroding, EDM is a nontraditional method of removing material by a series of rapidly recurring electric arcing discharges between an electrode (the cutting tool) and the work piece, in the presence of an energetic electric field (applied potential). The EDM cutting tool is guided along the desired path very close to the workpiece, but it does not touch the workpiece. Consecutive sparks produce a series of micro-craters on the work piece and remove material along the cutting path by melting and vaporization. The particles are washed away by the continuously flushing dielectric fluid.
Electro Chemical Machining (or ECM) is a method of working extremely hard materials or materials that are difficult to machine cleanly using conventional methods. It is limited, however, to electrically conductive materials. ECM can cut small or odd-shaped angles, intricate contours or cavities in extremely hard steel and exotic metals such as titanium, hastelloy, kovar and inconel.
ECM is similar in concept to EDM in that a high current is passed between an electrode and the part and through an electrolyte. While the applied potential in EDM ranges from 20 to 200V, the applied potential in ECM is lower and ranges from a few mV to about 30V. The ECM cutting tool is guided along the desired path very close to the work but it does not touch the workpiece. Unlike EDM however, no sparks are created. The workpiece is corroded away by the electro-chemical reaction occurring at the surface of the workpiece. Very high metal removal rates are possible with ECM, along with no thermal or mechanical stresses being transferred to the part, and mirror surface finishes are possible. The ECM process is most widely used to produce complicated shapes with good surface finish in difficult to machine materials, such as turbine blades. It is also widely used as a deburring process.
Electro Chemical Grinding (or ECG) is a combination of electrochemical (Anodic) dissolution of a material, according to Faraday's Law, and light abrasive action. The metal is decomposed to some degree by the DC current flow between the conductive grinding wheel (Cathode) and the work piece (Anode) in the presence of an electrolyte solution.
Electrochemical oxidation and reduction occurs on the surface of electrodes when an electric current is passed between the electrodes through an electrolyte fluid. An electrochemical potential between the electrodes causes current to flow from the anode to the cathode in the DC circuit. In ECG, the anode is the workpiece, and the cathode is the conductive grinding wheel. A continuous stream of electrolyte flows at the interface of the grinding wheel and work piece and conducts the current in the circuit. The electrolyte fluid is often a conductive aqueous solution consisting of a mixture of chemical salts and other additives. At the positive electrode, or anode, oxidation of the work piece dissolves the surface, and in many cases resulting in the formation of an oxide film. The film is electrically insulating, and acts as a barrier against the electrochemical cutting action of the process.
The abrasives in the rotating grinding wheel continually remove this film and expose a fresh surface for oxidation. Metal deposition on the grinding wheel (cathode) is avoided by proper choice of electrolyte. Dissolution of the metal, combined with the mechanical removal of the oxides, results in an efficient low-stress cut.
Electro Chemical Discharge Machining (or ECDM) using DC or pulse voltage and Electro Chemical Arc Machining (ECAM) using constant or pulse voltage are the combined methods of machining involving ECM and EDM. A combination of ECM with EDM in one process, ECDM in an electrolyte solution has shown to contain the benefits of both processes, provided that the parameters of the combined process are properly selected.
In a high-speed electro-erosion (HSEE) process, controlled instantaneous short circuits between the electrodes are utilized for high speed metal erosion and improved tolerance control. Due to the effect of electrical erosions on the machining surface, a large number of craters are formed on the surface. These surface irregularities are subject to the crater size and density control. It has been shown that the process yields rates of material removal using the HSEE process can be as much as five to fifty times greater than ECM and EDM, respectively.
However, some advanced difficult-to-machine materials, such as titanium and titanium alloys, and stringent design requirements still pose major problems in the manufacturing industry. Titanium remains difficult to machine by HSEE due to lower thermal conductivity and inconsistent molten chip ejection during cutting as compared to machining other alloys, such as nickel-based alloys and the like. To meet these challenges, innovative techniques or modifications of the existing methods is needed.